Method and device for measuring the thickness of thin films near a sample&#39;s edge and in a damascene-type structure

ABSTRACT

A method for measuring a structure that contains overlying and underlying films in a region where the overlying film&#39;s thickness rapidly decreases until the underlying film is exposed (e.g., an edge-exclusion structure). The method includes the steps of: (1) exciting acoustic modes in a first portion of the region with at least one excitation laser beam; (2) detecting the acoustic modes with a probe laser beam that is either reflected or diffracted to generate a signal beam; (3) analyzing the signal beam to determine a property of the structure (e.g., the thickness of the overlying layer) in the first portion of the region; (4) translating the structure or the excitation and probe laser beams; and (5) repeating the exciting, detecting, and analyzing steps to determine a property of the structure in a second portion of the region.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/067,411, filed Apr.27, 1998, now U.S. Pat. No. 6,256,100.

BACKGROUND

This invention relates to methods for determining the thickness of thinfilms on small areas of structures used in microelectronics fabrication,e.g., near a semiconductor wafer's edge or on a damascene-typestructure.

During the fabrication of microelectronic devices, thin films of metalsand metal alloys are deposited on silicon wafers and for use aselectrical conductors, adhesion-promoting layers, and diffusionbarriers. Microprocessors, for example, use metal films of copper,tungsten, and aluminum as electrical conductors and interconnects,titanium and tantalum as adhesion-promoting layers, and titanium:nitrideand tantalum:nitride as diffusion barriers. Thickness variations inthese films can modify their electrical and mechanical properties,thereby affecting the performance of the microprocessor. The targetthickness values of these films vary depending on their function:conductors and interconnects are typically 3000-10000 angstroms thick,while adhesion-promoting and diffusion-barrier layers are typicallybetween 100-500 angstroms thick. The deposition of each of these filmsmust be controlled such that the film's thickness is within a fewpercent (e.g., 5-100 angstroms, a value roughly equivalent to one or twoseconds human fingernail growth) of its target value. Furthermore, theuniformity of the film over the surface of the wafer must be closelycontrolled in order to assure uniform behavior of the individualmicroprocessors and, consequently, high manufacturing yields. Because ofthese rigid tolerances, film thickness is often measured as aquality-control parameter during and/or after the microprocessor'sfabrication.

The metal films are often deposited and patterned in complex geometriesand this complicates the measurement process. In a typical fabricationprocess, a titanium:nitride film is deposited over the entire surface ofa silicon wafer. A tungsten film is then deposited onto thetitanium:nitride film to leave an “edge-exclusion zone”, i.e., a small(about 1 or 2 mm) region where the titanium:nitride is exposed, near thewafer's edge. The edge-exclusion zone prevents delamination of thetungsten film neat its edges. Near this region, the thickness of thetungsten film rapidly increases to its target value; this takes placeover a distance of a few hundred microns. Without this rapid increase infilm thickness, devices patterned near the wafer's edge-exclusion zonewill contain non-ideal tungsten films not having adequate thickness, andthey will not meet specifications.

An example of a complicated film geometry recently introduced incommercial microelectronics fabrication is a “damascene” or “dualdamascene” structure. These structures, used especially to form copperconductors and interconnects, are typically formed by a multi-stepprocess: copper is first deposited onto a wafer having a dielectriclayer that has been etched to have a series of trenches; the wafer isthen polished by chemical-mechanical polishing (CMP) to remove excesscopper, leaving only copper-filled trenches. The resulting structure istypically a series of separated copper lines having a thickness of a fewthousand angstroms, a width of about 0.5 microns, a periodicity of about2 microns, and a length of several millimeters.

Measuring film thickness in and near the edge-exclusion zone and indamascene-type structures is difficult and impractical usingconventional techniques. For example, blanket metal films are typicallymeasured using a 4-point probe. Here, two separated pair of conductingprobes contact the film; electrical resistance, as measured by theprobes, relates to the film's thickness. Because the spatial resolutionof the 4-point probe is typically a few hundred millimeters, thisinstrument is impractical for both edge-profile and damascene-typestructures. Moreover, a film's resistance often depends on both itsthickness and geometry, a complication that further reduces the accuracyof the 4-point probe when used to measure complex geometries. Anotherfilm-measuring instrument, called a stylus profilometer, drags a stylusneedle over a sample, recording variations in topography. Thisinstrument, however, is slow, cumbersome, sensitive to slight amounts ofsample curvature, and inaccurate when used to measure relatively longdistances (e.g., the hundreds of microns required for tungsten build-upnear the exclusion zone).

In addition to these disadvantages, both 4-point probes and stylusprofilometers require contacting and thus contaminating a sample. Theseinstruments are therefore typically used on monitor or test samples,rather than samples containing actual product. Other methods formeasuring the thickness of metal films, such as X-ray fluorescence andRutherford backscattering, are non-contact, but are slow and have poorspatial resolution.

SUMMARY

In general, in one aspect, the invention provides a method for measuringa structure that contains overlying and underlying films in a regionwhere the overlying film's thickness rapidly decreases until theunderlying film is exposed (e.g., an edge-exclusion structure). Themethod includes the steps of: (1) exciting acoustic modes in a firstportion of the region with at least one excitation laser beam; (2)detecting the acoustic modes with a probe laser beam that is eitherreflected or diffracted to generate a signal beam; (3) analyzing thesignal beam to determine a property of the structure (e.g., thethickness of the overlying layer) in the first portion of the region;(4) translating the structure or the excitation and probe laser beams;and (5) repeating the exciting, detecting, and analyzing steps todetermine a property of the structure in a second portion of the region.

In one embodiment, the exciting, detecting, analyzing, and translatingsteps are repeated to determine a property of the structure in multipleportions of the region. In one case, the above-mentioned steps arerepeated in an edge-exclusion structure until the thickness of theoverlying film is measured from where the underlying film is exposed towhere the overlying film's thickness is at least 80% of its averagevalue. This particular method can be extended so that the steps arerepeated in the structure until a diameter of the overlying film ismeasured. Typically, this “diameter scan” embodiment includes repeatingthe above-mentioned step on each side of the overlying film's diameter,and measuring multiple points near the center of the film.

In another embodiment, the exciting, detecting, analyzing, andtranslating steps are repeated until a property of the underlying film(e.g., the width of the edge-exclusion zone) is measured from where itis exposed to the edge of the wafer.

In typical embodiments: the overlying film is selected from a metal suchas tungsten, copper, aluminum, and alloys thereof; the underlying filmis selected from materials such as oxides, polymers, and metals such astitanium, titanium:nitride, tantalum, tantalum:nitride, and alloysthereof. These films are usually deposited on a silicon wafer.

The structure is typically measured using an optical method where theacoustic modes are excited with at least one optical pulse having aduration less than 1 nanosecond. In a particular embodiment, theexciting step features exciting time-dependent acoustic modes in thestructure by directing a spatially periodic excitation radiation fielddefined by a wavevector onto the sample. The radiation field, forexample, is formed by overlapping two optical pulses in time and spacein or on top of the sample. The detecting step then includes diffractingprobe radiation off a modulated optical or mechanical property inducedon the sample's surface by the acoustic modes. To determine thickness ofthe overlying layer, the density and acoustic properties of theoverlying and underlying layers, the wavevector, and a frequency of theacoustic mode are analyzed (e.g., by comparing them to a mathematicalmodel).

In another aspect, the invention features a method for measuring astructure comprising multiple thin, metallic, rectangular-shaped orlinear regions, each having a width of less than 5 microns and beingdisposed between neighboring regions that include a second, non-metallicmaterial (e.g., a damascene-type structure). The method includes thesteps of: (1) exciting acoustic modes in at least one metallic,rectangular-shaped region by irradiating the region with a spatiallyperiodic excitation field defined by a wavevector; (2) detecting theacoustic modes by diffracting a probe laser beam off a ripple morphologyinduced in the regions by the acoustic modes; and (3) analyzing thediffracted signal beam to determine a property of the structure (e.g.,the thickness of the metallic, rectangular-shaped regions).

In a particular embodiment, the exciting step includes irradiatingmultiple metallic, rectangular-shaped regions with the excitation field.A probe laser beam is then diffracted off the surface ripple induced ineach region by the acoustic modes. Thickness can be determined byanalyzing a density and acoustic properties of the metal included in theregion, the wavevector, and a frequency of the acoustic mode. Here, thewidth of the metallic, rectangular-shaped region or a distanceseparating consecutive regions may be used in the analysis. In stillother embodiments, the signal beam can be further analyzed (e.g., bymonitoring diffraction of the probe beam) to determine a width of themetallic, rectangular-shaped region or a distance separating consecutivemetallic, rectangular-shaped regions.

In embodiments, each of the metallic, rectangular-shaped regionscomprises copper, tungsten, aluminum, or alloys thereof, and have awidth of less than 1 micron. The rectangular-shaped regions can alsoinclude more than one layer. For example, the trench may be lined withtantalum and then filled with copper.

In another aspect, the invention provides a method for measuring astructure comprising multiple thin, metallic, rectangular-shapedregions, each having a width of less than 1 micron and being disposedbetween neighboring regions comprising a second, non-metallic material.The method includes the steps of: (1) exciting acoustic modes inmultiple metallic, rectangular-shaped regions by simultaneouslyirradiating the regions with a spatially periodic excitation fielddefined by a wavevector; (2) detecting the acoustic modes by diffractinga probe laser beam off a modulated optical or physical property inducedin each of the regions by the acoustic modes; and (3) analyzing thesignal beam to determine an average thickness of the metallic,rectangular-shaped regions irradiated by the excitation field.

The invention has many advantages. In particular, the method makesaccurate measurements of film thickness in and near the edge-exclusionzone, in damascene-type structures, and in other small-scale structures.Measurements feature all the advantages of optical metrology, e.g.,noncontact, rapid and remote measurement over a small region. The methodcollects data from a single measurement point having an area of between10 and 100 microns in less than a few seconds. From these data filmthickness in the small-scale structures is determined with an accuracyand repeatability of a few angstroms. For damascene-type structures, themethod simultaneously measures the thickness multiple metal lines lyingwithin the optical spot size with virtually no decrease in data quality,accuracy, or repeatability. For typical films used in a microelectronicdevice, the measurement determines thickness to within a fraction of apercent of the film's true value.

In addition to thickness, the measurement determines the width of anexclusion zone, the diameter of the useable area on the wafer, thefilm's slope near the edge-exclusion zone, and properties ofdamascene-type structures, such as the width and periodicity of themetal lines and the number of defects in the structure.

The optical system used to make these measurements is compact, occupyinga footprint of about 2 square feet, and composed primarily ofinexpensive, commercially available components.

Because of its small size, the optical instrument can be a stand-aloneunit, or can be attached directly to a film-formation tool (e.g., achemical-vapor deposition tool, a plasma-vapor deposition tool, acluster tool, or a vacuum chamber) or a film-processing tool (e.g., achemical-mechanical polisher). In these embodiments, the film-formationtool includes an optical port (e.g., a glass window) that is transparentto the excitation and probe radiation. Thus, during operation, thefilm-measuring instrument is oriented so that the excitation and proberadiation, and the diffraction signal, pass through the optical port.

Other features, aspects, and advantages of the invention follow from thefollowing detailed description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing showing an optical beam configuration ofthe optical system according to the invention;

FIG. 1B is a top view of excitation and probe beams of FIG. 1A;

FIG. 2 is a plot (signal intensity vs. time) of a signal waveformmeasured from a copper film using the optical beam configuration of FIG.1A;

FIG. 3 is a flow chart describing a method for determining filmthickness by analyzing data taken from an edge-exclusion structure;

FIG. 4 is a cross-sectional, schematic drawing of an edge-exclusionstructure;

FIG. 5 is a plot of tungsten film thickness as a function of distancemeasured using ISTS in an edge-exclusion structure;

FIG. 6 is a plot of tungsten film thickness as a function of distancemeasured across a diameter of a silicon wafer structure using ISTS;

FIG. 7A is a top view of a damascene-type structure being measured withISTS;

FIG. 7B is a cross-sectional view of the damascene-type structure ofFIG. 7A;

FIG. 8 is a plot (signal intensity vs. time) of a signal waveformmeasured from a copper damascene-type structure similar to that shown inFIGS. 7A and 7B;

FIG. 9 is a schematic drawing of an optical system and an opticaldetection system for measuring a small-scale structure according to themethod of the invention;

FIG. 10 is a schematic drawing of an optical system, optical detectionsystem, computer, and signal processor for measuring a small-scalestructure according to the invention; and

FIG. 11 shows a flow chart for a computer-implemented algorithm fordetermining the thickness in Damascene-type structures.

DETAILED DESCRIPTION

In FIGS. 1A, 1B, and 2, a thickness of a thin film 10 disposed on asubstrate 12 is measured in a small area 13 (e.g., in a damascene-typeor edge-profile structure) when two excitation laser pulses 16, 16′ anda probe laser pulse 18 irradiate the film. The excitation pulses 16, 16′are short in duration (e.g. about 0.5 nanoseconds), have a wavelengththat is absorbed by the film, and are separated by an angle α. The probepulse 18 is relatively long (e.g. several hundred nanoseconds or longer)and has a wavelength that is not strongly absorbed by the film. In thisconfiguration, called a “four-wave mixing” geometry, the excitationpulses 16, 16′ overlap in time and space and interfere to form aspatially and temporally varying excitation radiation field 14 in or onthe surface of the film 10. The field 14 is composed of a series ofperiodic, sinusoidal “bright” regions 14 a (i.e., constructiveinterference) and “dark” regions 14 b (i.e., destructive interference).The length and width of the field 14, shown by the arrows 15 a and 15 b,are about 500 and 40 microns, respectively. When focused onto the film,the probe pulse 18 forms a second field 17 that is elliptical (roughly90 microns by 25 microns) and lies completely within the excitationfield 14.

The direction of the excitation field is defined by a wavevector that isinversely proportional to the spatial distance between consecutivebright (or dark) regions. The magnitude (q) of the wavevector isdetermined by the angle α between the excitation pulses and thewavelength λ₁ of each pulse using the equation q=4π sin(α/2)(λ₁)⁻¹=2π/Λ,where Λ is the grating wavelength.

The excitation radiation field 14 excites acoustic modes in the film 10that have a wavelength and orientation corresponding to the excitationwavevector. Excitation of the acoustic modes occurs via ImpulsiveStimulated Thermal Scattering (“ISTS”), a four-wave mixing techniquethat is described in detail in U.S. Pat. No. 5,633,711 (entitledMEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S.Pat. No. 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILMSAMPLES), and U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FORMEASURING FILM THICKNESS, filed Jul. 15, 1996), the contents of whichare incorporated by reference. The acoustic modes induce a modulatedoptical and physical property in the film (e.g., a time-dependent“ripple” morphology and/or time-dependent refractive index change). Inthe case of an induced time-dependent ripple morphology, this can beobserved on the film's surface. The frequency of the modulation dependson the thickness of the film. Acoustic modes are detected by diffractingthe probe pulse off the modulated property to form at least two signalbeams 20, 20′ disposed on each side of a reflected probe beam 18′. Aphotodetector detects one or more of the signal beams to generate asignal waveform 30 similar to that shown in FIG. 2 which presents datataken from a nominal 3000 angstrom copper film. The Fourier transform ofthe signal waveform 30, indicated by the graph 35 in the figure inset,indicates the frequency of the acoustic mode. To determine filmthickness, the frequency is analyzed along with the wavevector and thefilm's density and sound velocities, as described in the above-mentionedreferences.

Measurement of Small-Scale Structures

Accurate measurement of small-scale structures using ISTS depends on: 1)carefully choosing the optical properties of the excitation and probepulses in the four-wave mixing geometry to obtain a data signal thatallows one to make the measurements (e.g., a data signal with sufficientsignal-to-noise ratio, amplitude, and number of excited modes); and 2)modifying the methods for calculating film thickness described in U.S.Ser. No. 08/783,046, entitled METHOD AND DEVICE FOR MEASURING FILMTHICKNESS, filed Jul. 15, 1996) to account for the physical geometrysmall-scale structures.

Optical properties that can be adjusted include the wavevector (andhence the acoustic frequency and wavelength), orientation, and opticalwavelength of the excitation and probe field. For example, whenmeasuring a damascene-type structure, the long axes of both theexcitation and probe fields are oriented along the extension of themetal lines. This ensures that the ISTS-generated acoustic modespropagate along the extension of the metal lines, and minimizescontributions to the signal arising from reflections at the sides of themetal lines. To measure an edge-exclusion structure, multiplemeasurements are made along a line that extends up the rising edge ofthe film. In this case, the long axes of the excitation and probe fieldsare typically oriented perpendicular to that line (i.e., tangent tp theedge), and the short axes of the fields are oriented along the line.This configuration minimizes acoustic reflections that may occur nearthe edge of the film, and ensures the highest possible spatialresolution.

In addition, the wavevector of the excitation field is selected toproduce accurate measurements of the above-mentioned small-scalestructures. The wavevector is adjusted by changing the angle (α in FIG.1A) separating the excitation beams. When measuring an edge-exclusionstructure, for example, the selected wavevector is one that launchesacoustic waves at a frequency that is sensitive to changes in overlyingfilm thickness, but relatively insensitive to changes in the underlyingfilm thickness. This methodology can be applied to edge-exclusionstructures where tungsten and titanium:nitride are used, respectively,as the overlying and underlying films. Here, a wavevector between200,000-700,000 m⁻¹ results in acoustic frequencies that are typicallysensitive to chances in the tungsten film thickness, but insensitive tothickness changes in the titanium:nitride thickness.

For the damascene-type structures, the selected wavevector is one thatgenerates an acoustic wavelength that is relatively long compared to thewidth of the metal lines, which are typically on the order of a micronor less. This ensures that the width of the lines has a minimal effecton acoustic modes propagating inside.

The optical wavelength of the excitation and probe pulses are alsoselected to produce accurate measurements of the small-scale structuresdescribed above. In the case of damascene-type structures, theexcitation wavelength is chosen to be absorbed by the metal lines, andnot absorbed by the surrounding structure (typically silicon dioxide).Since copper is typically used for the metal lines in damascene-typestructures, an effective wavelength is in the visible portion of thespectrum, e.g., 532 nm. Wavelengths that are not useful include those inthe infrared (e.g., 1064 nm) which are highly reflected by the copper.When measuring the edge-exclusion structure the wavelength is chosen sothat the light is absorbed by both the overlying film (e.g., tungsten)excluded from the edge, and the underlying film (e.g., titanium nitride)exposed in the edge-exclusion zone. An appropriate wavelength for thiscase is 1064 nm. Using this wavelength, ISTS measures the risingproperties of the overlying film, the thickness of the underlying film,and the width of the edge-exclusion zone.

FIGS. 3 and 4 show, respectively, a flow chart describing a method 47for analyzing data collected from a typical edge-exclusion structure,and a schematic drawing showing the structure 60 where the measurementsare made. The structure 60 includes an overlying film 62 (typicallytungsten or copper) separated from an edge 66 of a silicon wafer 68 byan edge-exclusion zone 69. The overlying film is deposited orelectroplated onto an underlying film 64 (typically titanium, tantalum,or alloys thereof) that extends to the edge 66 of the wafer 68. Theoverlying film 62 decreases from its maximum thickness value (typicallya few thousand angstroms) to 0 angstroms along a “build-up region” 70 ofthe film. The build-up region 70 typically has a length of a few hundredmicrons that extends from the region separating: 1) a point where theoverlying film's thickness is greater than about 80% of the film'smaximum thickness; and 2) an endpoint 72 where the overlying film has 0thickness. In the edge-exclusion zone 69, the underlying film 64 isexposed from the endpoint 72 to the wafer's edge 66.

To correctly determine film thickness, the material properties (i.e.,density and sound velocities) of the overlying and underlying films areused in the above-mentioned calculations along with the wavevector andacoustic frequency measured in the small-scale structure. In oneembodiment, the overlying film is measured by first selecting the film'smechanical properties and using these in the thickness calculation (step48). Measurements are then made starting at a point near the maximumthickness of the overlying film, through the build-up region, and ontothe film's endpoint (step 50). This measurement path is indicated inFIG. 4 by the arrow 78. Since measurements along an edge-exclusion zoneare typically performed using an automated instrument, the analysismethod must detect when the underlying film's endpoint is reached (step52). Both the time-dependent shape and acoustic frequency of the signalwaveform will change abruptly when this happens, and thus theseproperties can be used to detect this position. This can be done, forexample, by analyzing the time-dependent shape of the signal waveform,or by finding an abrupt change in the signal waveform's frequency. Oncethe endpoint is reached, the mechanical properties used in the thicknesscalculation are switched to those of the underlying film (step 54).These properties are then used to calculate thickness using data frommeasurements made throughout the exclusion zone (step 56). Datacollection is stopped once the wafer's end is reached (step 58); thisregion is best indicated by the absence of a signal waveform. Theresulting data are then analyzed to determine properties such asoverlying and underlying film thickness, length of the build-up region,and the width of the edge-exclusion zone (step 60).

The method described above can also be extended to measure filmthickness along other regions of a wafer. For example, measurements canbe made along the wafer's diameter to include: 1) the thickness of across-sectional slice of the wafer; and 2) the thickness ofedge-exclusion structures on each side of the cross-sectional slice.These measurements are particularly useful, as they indicate a “usable”portion of the film that has an adequate thickness (e.g., 95% of thefilm's mean thickness value), and can therefore be used to qualifydevice-fabrication processes.

The method for determining the thickness of metal lines included indamascene-type structures is similar to that described in U.S. Ser. No.08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS,filed Jul. 15, 1996). For this application, the method is modified toinclude the geometry of the metal lines in the calculation.Specifically, in addition to the metal used to form the metal lines(copper is typically used in this application) the line width,periodicity, and cross-sectional geometry are taken into account in thecalculation. In essence, the thickness of the copper bars in adamascene-type structures are measured by initiating acoustic wavesalone the long dimension of the bars using ISTS, and then sampling thesewaves with a probe beam.

FIGS. 5 and 6 show, respectively, data collected from an edge-exclusionstructure and across the diameter of a tungsten/titanium:nitride/silicondioxide/silicon structure. The underlying titanium:nitride and silicondioxide films had, respectively, thicknesses of 1100 and 5500 angstroms.In this case, the wavevector of the excitation field (698,131 m⁻¹) isset so that the ISTS-initiated acoustic waves have a frequency that issensitive to changes in the tungsten film thickness, but relativelyinsensitive to changes in the titanium:nitride thickness. This ispossible because tungsten has a relatively high density (19,300 kg/m³)compared to that of titanium:nitride (5400 kg/m³); small thicknessvariations in tungsten result in significant mass-loading of thesilicon, and are therefore relatively easy to measure. The excitationwavelength for these data was 1064 nm, and the short axes of theexcitation and probe spots are orientated along the direction that thewafer is translated.

FIG. 5 shows the properties of the tungsten and titanium:nitride filmsin the edge-exclusion structure. As indicated by shape of a rising edge112, the tungsten film starts building up at a distance of about 74.22mm from the wafer center, reaching a thickness of 3870 angstroms at adistance of 65.00 mm from the wafer's center. The smooth, systematicbuild-up indicates the precision of the measurement, which is estimatedto be on the order of a few angstroms. The data also indicate atitanium:nitride exclusion zone 118 a that has a width of about 660microns. FIG. 6 shows similar data taken across a diameter of the samewafer. Here, the curve 110 includes data indicating rising edge 112, afalling edge 114 near the edge-exclusion zones 118 a, 118 b at thewafer's edges. These regions bracket the center portion 116 of the filmthat has a relatively constant film thickness. The arrow 120 in thefigure indicates a portion of the film (in this case 148.8 mm) that isgreater than 80% of the film's maximum thickness value, and is thereforeconsidered to be “usable” for fabrication purposes.

FIGS. 7A and 7B indicate how film-thickness measurements are made in adamascene-type structure 120. Here, the structure 120 includes a seriesof metal lines 122 deposited into trenches 123 etched into a silicondioxide film 124. Metal lines in this type of structure are typicallymade of copper and have a width less than 1 micron and a length ofseveral millimeters. During ISTS, the excitation field 14 is typicallyoriented with the long axis extending parallel to the metal lines 122.Thus, the dark 14 a and light 14 b regions of the field 14 are spacedperiodically along the lines. As indicated by the arrows 125, thisgenerates acoustic modes that propagate along the metal lines. Acousticwaves are initiated in and coherently propagate along multiple linessince the width of the excitation field 14 is larger than the width of asingle line. These modes are collectively measured by irradiatingmultiple lines with a probe field 17. The field 17 is diffracted off thesurface ripple in the irradiated lines. The diffracted signal is thendetected and analyzed in combination with copper's material propertiesand the width, periodicity, and cross-sectional geometry of the metallines to determine the film thickness.

FIG. 8 shows time-dependent 130 and frequency-dependent 135 datameasured from a copper damascene-type structure using the above-describemethod. The structure included copper metal lines having a thickness of8000 angstroms, a width of 1 micron, periodicity of 2 microns, and arectangular cross section. The data have a high signal-to-noise ratioand acoustic damping similar to that shown in FIG. 3, indicating thatacoustic modes excited in the copper damascene-type structure diffractthe probe field with an efficiency that is comparable to that achievedfrom non-patterned copper films. The repeatability of these data wasabout 0.2 MHz, indicating that film thickness can be measured with arepeatability of better than 10 angstroms.

Once measured, the acoustic velocity is processed using acomputer-implemented algorithm that determines how the damascenestructure “mass loads” the underlying silicon substrate. In general, themass-loading algorithm determines the total mass of the overlyingdamascene structure (i.e., the mass of the copper and oxide layers).Thickness is then determined using the density of the copper and oxidematerials. Mass loading increases with the mass of the overlyingstructure. In other words, an increase in mass decreases the velocity ofthe acoustic wave. The velocity depends on i) the film thickness and ii)the width and periodicity of the damascene bars. As described elsewhere,these latter properties can be determined by monitoring the reflectedand diffracted probe beams that leave the sample. (Note: thesediffracted beams are different from the signal beams).

The general algorithm for determining copper film thickness fromdamascene-type structures is described by the steps listed below. Theflow chart presented in FIG. 11 summarizes these steps.

Step 1: Determine the acoustic wave velocity or frequency using ISTS.The wavelength of the acoustic wave is preferably long (e.g. typically10-15 microns) compared to the width of the damascene pattern, and theacoustic waves propagate along the long dimension of the copper bars.

Step 2: Determine the total amount of mass (M) in the overlyingdamascene structure (oxide and copper films) using the followingequation:

M=(ν−ν_(o))/kc _(o)

where ν_(o) is the known Rayleigh wave velocity of the substrate, c_(o)is a elastic constant of the substrate that is known or can be measuredexperimentally, k is the acoustic wavevector, and ν is the acousticvelocity measured from the structure using ISTS.

Step 3: Determine the width and periodicity of the bars in the damascenestructure. This is typically well-known for the structure beingmeasured, but can also be determined by measuring the diffracted andreflected beams leaving the sample. Specifically, the periodicity isrelated to the angle (θ) of the diffracted beams leaving the sampleusing the equation d·sin θ=nλ, where d is the bar periodicity, n is thediffracted order, and λ is the optical wavelength. The reflectivity ofthe incident radiation can be analyzed to determine the bar width.

Once determined, the bar width defines the fraction of the damascenestructure that is copper (f_(Cu)) and the periodicity defines thefraction that is oxide (f_(SiO2)). Note that f_(Cu)+f_(SiO2)=1.

Step 4: Assuming the thickness of the oxide and copper are the same, thethickness (t) of these layers is given by:

t=M/(f _(Cu)ρ_(cu) +f _(SiO2)ρ_(SiO2))

where ρ_(cu) and ρ_(SiO2) are, respectively, the densities of the copperand oxide films.

Optical System for ISTS Measurements

A suitable optical system for performing the ISTS measurements isdescribed in U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FORMEASURING FILM THICKNESS, filed Jul. 15, 1996), the contents of whichhave been incorporated herein by reference.

FIGS. 9 and 10 show particular optical systems 200 for making four-wavemixing measurements on small-scale structures. The system 200 featuresan excitation laser 201 that generates a single excitation beam 205. Theexcitation laser is typically a pulsed Nd:YAG laser that is Q-switched,mode-locked, or both. A particularly desirable excitation light sourceis a microchip laser, such as that described in U.S. Pat. No. 5,394,413.The pulse duration of the excitation beam 205 must be short enough toimpulsively stimulate material motions in the film via ISTS, and istypically about 0.5 nanoseconds. The output pulse typically has anenergy of about 10 microjoules, and a wavelength of 1064 or 532 nm.

During operation, the single excitation beam 205 the single excitationbeam 205 passes through a partially reflecting optic 209 that reflects aportion of the excitation beam 205 into a triggered detector 221. Asshown in FIG. 10, the trigger detector 221 sends an electrical signal230 to a computer 231 that processes data generated by the opticalsystem 200. The single excitation beam 205 then passes through anattentuating neutral density filter 213 and is then focused onto a phasemask 204 using a cylindrical lens 206. The phase mask 204 includes apattern 207 that diffracts the incident excitation beam into twospatially diverging beams 205 a,b, each having a pulse duration of about0.5 ns. The angle of divergence θ is determined by the diffractingpattern 207 on the phase mask 204. Once diffracted, the diverging beams205 a, 205 b are imaged onto a sample 210 using an imaging lens 212.This forms an excitation radiation pattern (14 in FIG. 1) having awell-defined wavevector. The excitation pattern excites acoustic modeson the sample's surface. Excitation patterns having differentwavevectors are formed simply by translating the phase mask to movedifferent diffracting patterns into the excitation beam's path.

A probe laser 202 (typically a single-mode diode laser producing between0.1 and 1 Watt in the visible or infrared frequency range) in theoptical system 200 generates a laser beam 216 that is focused to a field(17 in FIG. 1B) that is smaller than the spot size of the excitationradiation field using a focusing lens 220. Once focused, the beam 216 isdiffracted to form a pair of signal beams 216 a, 216 b. One of thesignal beams 216 a is then directed onto an optical detection system224, while the reflected pulse beam 216′ is blocked. The opticaldetection system 224 contains a photodetector 225 that has a highbandwidth (e.g., 1 GHz) to time resolve the individual features of thesignal beam. In this way, the entire temporal duration of the acousticmodes can be sent to a signal processor 240 and measured in real time(e.g., 100 nanoseconds) to generate a signal waveform 241. The waveformsignal-to-noise ratio can be enhanced by signal averaging at a ratelimited only by the repetition rate of the laser (typically around 500Hz) or the speed of the recording electronics (typically on the order ofa millisecond). Signal averaging typically improves the signal-to-noiseratio to greater than 500:1. Data-collection times for averaged signalsare typically one or two seconds.

The signal waveform 241 is then processed by a computer 231 as describedabove to determine, e.g. the thickness of a metal film at a single pointon the sample 210. The computer 231 then sends a signal 245 to amotor-controlled mechanical stage 250 that moves the sample to a newlocation. The computer 231 then repeats the above-described process todetermine a thickness of the metal film at a second location. Under thecontrol of the programmed computer 231, this entire process is repeatedmultiple times to measure properties of the film across its edge, e.g.thickness.

Other Embodiments

Other embodiments are within the scope of the invention. For example,other optical, photoacoustic methods may be used to determine filmthickness in the structures described above. These methods includepulse-echo, beam-deflection, and other four-wave mixing methods forinitiating acoustic phonons. Examples of these methods are described in“Ultrasonic multilayer metal film metrology”, published in Solid StateTechnology (June, 1997) and “Real-time detection of laser-inducedtransient gratings and surface acoustic wave pulses with a Michelsoninterferometer”, published in Journal of Applied Physics (Nov. 15,1997), the contents of which are incorporated herein by reference.

In other embodiments, properties of damascene-type structures, such asthe width or spacing of the metal lines, are measured by analyzing theproperties of the diffracted probe or signal beams. For example, theangular position of the diffracted probe radiation can be analyzed todetermine the periodicity and width of the rectangular bars in thedamascene-type structure. More features of the damascene-type structureare determined by measuring the relative intensity of the diffractedorders. For example, these intensities can be analyzed to determine thethickness of the oxide regions surrounding the copper bars. Intensityand spatial variations in these orders can also indicate defects in thesample, such as missing or irregular-shaped bars. In addition, otherproperties of the diffracted signal beam, such as the signal componentdue to thermal diffusion and the damping of the acoustic frequency, canbe analyzed to determine properties (e.g., thickness, periodicity, andnumber of defects) of the damascene type structure.

The method and apparatus described above can also be applied to othersmall-scale structures, such as the “streets” (i.e., the areas betweendevices on a wafer), bond pads, and device features contained in apatterned semiconductor wafer.

In still other embodiments the optical system may be modified toincrease the quality of the data collected. Modifications include thosedescribed in U.S. Ser. No. 08/885,555 (entitled IMPROVEDTRANSIENT-GRATING METHOD AND APPARATUS FOR MEASURING MATERIALPROPERTIES, filed Jun. 30, 1997), the contents of which are incorporatedherein by reference. Other modifications include using heterodyneamplification to improve the measurement of weak signal beams.

Still other embodiments are within the scope of the following claims.

What is claimed is:
 1. A method for measuring a thickness of astructure, the structure comprising an overlying film and an underlyingfilm, positioned on a substrate, where a thickness of the overlying filmdecreases until a section of the underlying film is exposed, comprisingof: exciting an acoustic component corresponding to the overlying filmin a first portion of the region using at least one excitation laserbeam; detecting the acoustic component with a device that irradiates theregion with a probe to generate a signal beam; analyzing atime-dependent feature of the signal beam corresponding to the acousticcomponent, to determine a thickness of the overlying film in the firstportion of the region; translating the structure or the excitation andprobe beams; and repeating the exciting, detecting, and analyzing stepsto determine a thickness of the overlying film in a second portion ofthe region; wherein the exciting, detecting, analyzing and translatingare repeated until the thickness of the overlying film is measured fromwhere the underlying film is exposed.
 2. The method of claim 1 whereinthe signal beam comprises a diffracted beam.
 3. The method of claim 2,further comprising repeating the exciting, detecting, analyzing, andtranslating to determine a property of the structure in each of multipleportions of the region.
 4. The method of claim 3, wherein the analyzingfurther comprises comparing material properties of the overlying andunderlying films and the acoustic frequency and a wavevector to amathematical model to determine a property of the structure in the firstportion of structure.
 5. The method of claim 2, wherein the structurecomprises overlying and underlying films disposed on a semiconductorwafer, and the exciting, detecting, analyzing, and translating arerepeated until a property of the underlying film is measured from whereit is exposed to the edge of the wafer.
 6. The method of claim 5,further comprising determining the distance from where the underlyinglayer is exposed to the edge of the semiconductor wafer.
 7. The methodof claim 5, wherein the film is measured across a region until atime-dependent shape of the signal waveform changes, an acousticfrequency of the signal waveform changes, or both the timedependent-shape and the acoustic frequency of the signal waveformchange.
 8. The method of claim 2, wherein the overlying film is selectedfrom a metal consisting of tungsten, copper, aluminum, and alloysthereof.
 9. The method of claim 2, wherein the underlying film isselected from the group consisting of titanium, titanium:nitride,tantalum, tantalum:nitride, and alloys thereof.
 10. The method of claim2, wherein the exciting further comprises exciting the acousticcomponent with at least one optical pulse having a duration less than 1nanosecond.
 11. The method of claim 10, wherein the exciting furthercomprises exciting time-dependent acoustic components in the structureby directing a spatially periodic excitation radiation field defined bya wavevector onto the structure, and the detecting step comprisesdetecting the acoustic components by diffracting probe radiation off amodulated optical or physical property induced on the structure'ssurface by the acoustic components.
 12. The method of claim 11, furthercomprising determining a thick ness of the overlying or underlying layerby analyzing a density and acoustic properties of the overlying layer,the wavevector, and a frequency of the acoustic component.
 13. Themethod of claim 1, wherein at least a portion of the exposed underlyingfilm forms an edge profile of the structure.
 14. A method for measuringa thickness of a structure, the structure comprising an overlying filmand an underlying film, positioned on a substrate, where a thickness ofthe overlying film decreases until a section of the underlying film isexposed, comprising: exciting an acoustic component corresponding to theoverlying film in a first portion of the region using at least oneexcitation laser beam; detecting the acoustic component with a devicethat irradiates the region with a probe beam to generate a signal beam;analyzing a time-dependent feature of the signal beam corresponding tothe acoustic component, to determine a thickness of the overlying filmin the first portion of the region; translating the structure or theexcitation and probe beams; and repeating the exciting, detecting, andanalyzing steps to determine a thickness of the overlying film in asecond portion of the region; repeating the exciting, detecting,analyzing, and translating steps to determine a property of thestructure in each of multiple portions of the region; wherein the signalbeam comprises a diffracted beam; wherein the thickness of the overlyingfilm is the property determined in each of the multiple portions of theregion; and wherein the exciting, detecting, analyzing, and translatingare repeated until the thickness of the overlying film is measured fromwhere the underlying film is exposed to where the overlying film'sthickness is at least 80% of its mean value.
 15. The method of claim 14,wherein the exciting, detecting, analyzing, and translating steps arerepeated in the structure until: (a) the thickness of the overlying filmis measured on one side of the structure from where the underlying filmis exposed to where the overlying film's thickness is at least 80% ofits mean value; (b) the thickness of the overlying film is measuredwhere its thickness is greater than 80% of its mean value: and (c) thethickness of the overlying film is measured on an opposing side of thestructure from where the underlying film is exposed to where theoverlying film's thickness is at least 80% of its mean value.
 16. Themethod of claim 15, wherein the structure is a semiconductor wafer, andthe exciting, detecting, analyzing, and translating steps are repealeduntil the overlying films thickness is measured along a diameter of thewafer.
 17. An apparatus for measuring a structure comprising overlyingand underlying films in a region where the overlying film's thicknessrapidly decreases until the underlying film is exposed, comprising: atleast one excitation laser beam that irradiates a first portion of theregion to excite an acoustic mode; a probe laser beam that irradiatesthe first portion of the region and generates a signal beam; an analyzerthat analyzes the signal beam to determine a property of the structurein the first portion of the region; and a stage that translates thestructure or the excitation and probe laser beams so that a secondportion of the region at an edge of the structure is irradiated with theexcitation and probe laser beams; wherein said analyzer includes a meansfor comparing a property determined in the first portion of the regionwith a property determined in a second portion of a region by comparingthe signal beams, wherein if a signal representing the propertydetermined in the second portion of the region does not approximate asignal representing the property determined in the first region saidanalyzer indicates that an underlying film at the edge of the structureis exposed and such measurement is of the underlying film and not of thethickness of the overlying film.
 18. The apparatus of claim 17, whereinthe analyzer comprises a computer configured to translate the stage. 19.The apparatus of claim 17, wherein the analyzer comprises a computerconfigured to determine the property of the structure.
 20. The apparatusof claim 19, wherein the property is thickness.
 21. The apparatus ofclaim 19, wherein the structure is a semiconductor wafer, and thecomputer is configured to repetitively translate the stage until theoverlying film's thickness is measured along a diameter of the wafer.22. The apparatus of claim 21, wherein the overlying film is selcetedfrom a metal consisting of tungsten copper, aluminum, and alloysthereof.
 23. The apparatus of claim 22, wherein the underlying film isselected from the group consisting of titanium, titanium: nitride,tantalum, tantalum: nitride, and alloys thereof.
 24. The apparatus ofclaim 22, wherein the excitation laser beam comprises at least oneoptical pulse having a duration less than 1 nanosecond.
 25. A method formeasuring a structure including an underlying and overlying film and anedge, said method comprising: generating an excitation field having anarea having a long axis and a short axis; aligning the long axissubstantially parallel to the edge to excite an acoustic component;detecting the acoustic component by irradiating it with a probe laserbeam to generate a signal beam; analyzing a feature of the signal beamcorresponding to the acoustic component to determine a property of thestructure; translating the structure or the excitation and probe laserbeams; and repeating the excitation, detection, and analyzing steps todetermine another property of the structure so as to measure a thicknessof a structure having a same determined property to the edge until theunderlying film is exposed.
 26. A method for measuring at least oneproperty of a structure comprising overlying and underlying films formedon a substrate, said method comprising: generating an optical excitationfield characterized by a wavevector chosen to selectively excite eitherthe overlying film of the underlying film; exciting an acousticcomponent with the optical excitation field; detecting the acousticcomponent with a probe laser beam that irradiates the component togenerate a signal beam; and analyzing a time-dependent feature of thesignal beam corresponding to the acoustic component to determine aproperty of the overlying or underlying film so as to measure athickness of a structure having same determined property to an edge ofthe substrate until the underlying film is exposed.